The word "addition" is used when adding things together or when there is more of something.

In this first set of examples, an addition is an expansion of the original design of a building.

Tom put an addition onto his house.

We're adding an addition to the back of our house.

How much will the addition cost?

An addition can also be a new family member:

Le and Minh and expecting an addition to their family any day now. (an addition = a new baby)

Yolanda is hoping an addition to the family will make everyone happier.

Edgar was a welcome addition to the family as an adoptee from Guatemala.

The word "addition" is also commonly used in two different prepositional phrases, in addition and in addition to. These phrases function as a transition between two ideas:

In addition to being overweight, Pamela also has problems with her heart.

In addition to increased temperatures worldwide, climate change is having an effect on precipitation.

The process of creating the plans for the project was very time-consuming; in addition, it was costly, so we're going to try a new approach. (Click here for more examples of "in addition" and "in addition to."

We also use the word "addition" when talking about mathematical operations that involve adding numbers together to arrive at a total.

Most children learn addition when they are in first or second grade.

How old where you when you learned how to do addition? (Notice that the verb "do" is often used when talking about adding numbers.)

Sarah is very good at doing addition in her head.

The word "additional" is an adjective:

We need additional space for our business.

Taxes are an additional amount of money paid when making a purchase.

The school put an additional ten minutes onto every school day in order to increase test scores.